Preheat oven to 375°F. Line baking sheets with parchment paper, if desired.
In a stand mixer (or using a handheld mixer) cream together butter and sugar for about 3 minutes on medium speed. Add egg, egg yolk, and vanilla and blend until combined. Scrape bowl if needed.
1 cup unsalted butter,, 1 ½ cups sugar, 1 whole egg + 1 egg yolk, 1 teaspoon vanilla extract
In a medium bowl, mix the flour, cream of tartar, cinnamon, baking soda, and salt, until blended. Set aside.
2 ¾ cup all-purpose flour, 2 teaspoons cream of tartar, 1 teaspoon baking soda, 1½ teaspoons cinnamon, ¼ teaspoon salt
Add half of the dry ingredients to the creamed ingredients and gently mix on low until blended. Add remaining dry ingredients and mix until combined.
OPTIONAL: Cover and chill dough for 30 minutes (for a thicker, puffier cookie).
In a small bowl, mix together the Cinnamon Sugar coating.
¼ cup sugar, 2 teaspoons cinnamon
Roll dough into 2-inch balls and roll in the cinnamon sugar mixture twice to ensure they are evenly coated. A medium-size (also called #50) cookie scoop works well for 2-inch cookies. Place cookie balls 2 inches apart on a prepared (or ungreased) cookie sheet.
Bake for 8-10 minutes. Let cool on a baking sheet for 2 minutes. Remove and let cool completely on a wire rack.
Store in an airtight container for 4-5 days.